To produce a satisfactory heterojunction field effect transistor (HFET) capable of high output power under RF drive, it is necessary for the device to have a high breakdown voltage. Unfortunately, in HFETs having a narrow band gap material channel layer, such as InGaAs alloys, a high breakdown voltage is difficult to achieve, in part due to impact ionization resulting from electrons injected into the channel. Impact ionization is one of the major breakdown mechanisms, and can occur when hot electrons are injected into the channel. During injection, the electron can gain energy from the conduction band edge discontinuity. If the energy exceeds the impact ionization threshold energy, impact ionization may occur in the channel.
Impact ionization can be reduced by using higher band gap material for the channel. This, however, results in a corresponding reduction in efficiency of the device. Other attempts at increasing breakdown voltage include increasing the effective Schottky barrier height for the gate electrode, and reducing peak electric field at the drain edge of the gate.
Power in HFETs is also limited by the maximum current available from the device. One of the mechanisms that limits this current in an HFET is the scattering by the roughness at the heterojunction interfaces. In the channel layer of HFET structures, rough hetero-interfaces are often present due to practical technical constraints. Electrons suffer interface roughness scattering at the bad interface, resulting in lower current.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a HFET capable of high output power.
Another object of the invention is to provide a new and improved HFET having an increased breakdown voltage threshold.
And another object of the invention is to provide an HFET having reduced impact ionization.
Yet another object of the present invention is to provide an HFET having high current density.
A further object of the present invention is to provide an HFET with reduced interface roughness scattering.